I have read already quiet a lot about the conductivity of water/ fluids but i can not find a formula to calculate the resistance between probes in a fluid.
For example:
use-water-as-a-conductor-for-a-circuit-powered-by-1-5v-battery
In the answer Peter Bennett measured about 50.000kOhms.
I would like to know how I can calculate it and what the parameters are.
Parameters I guess so far:
– average distance of the probes
– copper surface exposed to the water
– conductivity of the water (amount of salt for example)
– volume of the water ??
– critical voltage or linear??
I will be doing some home experiments with copper/metal in water and measure the resistance with a small AC/DC current through the water. But I want to calculate it first and then see if I can reproduce the result.
Edit:
I did some small test with the answers below in mind. Especially the EDLC component was clearly measurable. I did a small test in about 100ml /3.5g salt solution. My findings:
AC: with 5cm /0.2mm probes i got at 5cm about 70Ohm. The amount of probe in the liquid did alter the value. The distance was a key factor, my measurement was not accurate enough to give me a function. Overal very consistent results.
DC: same probes i first tried with around 1.5v and did notice electrolysis at that point the resistance was low but increased rapidly because of the corroded probes. if I lowered the voltage over the probes to 0.5v the resistance was about 1kOhm what was higher than i expected and no visible electrolysis did occur. Please be careful, DC in a salt solution will generate chlorine gas do it in a well ventilated place.
I did the measurements over a voltage divider circuit, will do these again later with a more scientific method and logging. But i did them now to get an overview of what to expect.
I'm still interested in formulas to calculate before I do more tests.
Best Answer
Not an easy problem to solve without modeling software. You have probably noticed that water has a value of resistivity rather than resistance, stated in ohm-meters. This is because the ratio of voltage to current is proportional to the distance and inversely proportional to the cross section of the water.
Take a look at the two images below and you can begin to see the problem. The voltage distribution is shown in figure 12.4 and the current density in 12.5. The voltage varies between any two points in the current path, and the current density would also vary if the graph had higher resolution. Then consider that this is a model of a thin disc, and so there is a third dimension that would be present in real life to add complexity. The boundary might be conductive, which would change everything. Finally, the surface area of the conductor in the water will make a difference - the model uses point sources.
Your best bet is to place the water in a vessel of known geometry with electrodes of know geometry and do some modeling or some empirical testing. Bare copper may form a non-conductive oxide layer.